<div id="controls">
  <button id="vpt">Viewport is target</button>
  <button id="vps">Viewport is scrollable</button>
  <button id="vp">Viewport is not scrollbar</button>
  <br />
  <button id="start">start</button>
</div>
<div id="stage">
  <div>
    <div id="target">
      <div id="viewport">
        <div id="element">
          <p id="hi">hi</p>
        </div>
      </div>
    </div>
  </div>
</div>
